Generally, integrated circuits and other semiconductor devices are used in a variety of electronic applications, such as computers, cellular phones, personal computing devices, and many other applications. Home, industrial, and automotive devices, which in the past included only mechanical components, now have electronic parts that require semiconductor devices.
Semiconductor devices are manufactured by depositing many different types of material layers over a semiconductor workpiece or wafer, and patterning the various material layers using lithography. The material layers typically include thin films of conductive, semiconductive, and insulating materials that are patterned and etched to form integrated circuits (IC's). There may be a plurality of transistors, memory devices, switches, conductive lines, diodes, capacitors, logic circuits, and other electronic components formed on a single die or chip.
Lithography involves the transfer of an image of a mask to a material layer of a die or chip, also referred to as a wafer. The image is formed in a layer of photoresist, the photoresist is developed, and the developed photoresist is used as a mask during a process to alter the material layer, such as etching and patterning the material layer.
As feature sizes of semiconductor devices continue to decrease, as is the trend in the semiconductor industry, transferring patterns from a lithography mask to a material layer of a semiconductor device becomes more difficult, due to the scattering and the diffraction of the light used to expose the photoresist. The scattering of light at small feature sizes is known as projection optics flare. Projection optics flare is stray light produced by light scattering from lens inhomogeneities, contamination, and surface roughness in the optics of the lithography system. The amount of flare is inversely proportional to the square of the wavelength of the light, and has become a significant factor for Extreme Ultraviolet (EUV) lithography.
It is often desirable to measure the projection optics flare in a system. One method of measuring projection optics flare is known as the Kirk method. In the Kirk method, a “dose-to-clear” must be measured. Dose-to-clear is the amount of energy (dose) to which a photoresist must be exposed in order to dissolve the entire exposed area of the photoresist when the photoresist is developed.
The value for the dose-to-clear in a photoresist is traditionally determined by visually inspecting a series of Scanning Electron Microscope (SEM) resist images exposed at different doses. Visual inspection of the resist images is subjective, and the determined dose-to-clear may vary based on the person inspecting the images.
Other methods measure the contrast of the image and determine the dose-to-clear based on when the contrast in the images is equal to zero. Image contrast, however, is affected by differences in the photo-resist, under-layer coating material, post exposure baking conditions, and SEM settings (e.g., magnification, voltage, current, frame, etc.). Therefore, contrast methods may not yield satisfactory results under all experimental conditions.
As such, it is desirable to provide objective and widely applicable methods and systems for determining dose-to-clear in a lithographic process. Furthermore, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.